The invention disclosed and claimed herein generally pertains to design of the focusing cathode or filament geometry of a rotating anode X-ray tube. More particularly, the invention pertains to a cathode design that normalizes impact temperature along the focal spot length. Even more particularly, the invention pertains to a design of the above type which effectively varies the width of the X-ray tube focal spot, as a function of position along the length thereof, to normalize impact temperature over the focal spot length.
In a rotating anode X-ray tube, a beam of electrons is directed through a vacuum and across a very high voltage, on the order of 100 kilovolts, from a cathode to a focal spot position on an annular tungsten target track. X-rays are produced as electrons strike the focal spot on the target track, which is mounted on a disk-shaped anode rotated at high speed. However, the conversion efficiency of X-ray tubes is quite low, so that very little of the total power input, typically less than 1%, is converted to X-radiation. The remainder, in excess of 99% of the input electron beam power, is converted to thermal energy or heat. Accordingly, the effective management of heat is a major concern in X-ray tube design.
As used herein, the term xe2x80x9cimpact temperaturexe2x80x9d refers to the temperature of the target track within the focal spot, resulting from impacting electrons of the electron beam. In view of the thermal considerations referred to above, the impact temperature must not exceed the melting temperature of tungsten, at any point within the focal spot, to avoid damage to the target track. At present, this temperature constraint limits the maximum power that an X-ray tube can deliver in high current applications such as computed tomography (CT) and cine angiography. More particularly, if electric power applied to the cathode is increased, in order to increase electron emissions and thereby produce a higher output of X-rays, it may be necessary to enlarge the area of the focal spot. The increased number of impacting electrons are thereby spread over a greater area, to enhance cooling and maintain a specified level of loadability. (As used herein, loadability refers to the ability of the target track, within the focal spot, to tolerate a given quantity of heat.)
As is well known by those of skill in the art, while increasing focal spot size tends to improve loadability, such increase also reduces the quality of images produced by X-rays derived from the focal spot. Accordingly, the temperature limitation referred to above has, in the past, required a trade-off in the design of X-ray tubes. That is, increasing X-ray output could result in diminished image quality, and improvements in image quality could require reduced X-ray output.
The invention provides a method and apparatus for normalizing the impact temperature across an X-ray tube focal spot, as a function of length. In accordance therewith, the invention is directed to apparatus for producing X-rays which comprises an anode supported for rotation within the tube, an annular target track mounted upon the anode for rotation therewith, and a cathode spaced apart from the anode. The cathode comprises a filament and a cathode cup, which cooperatively project a beam of electrons onto the target track, within the focal spot, to generate X-rays. The filament and cathode cup are respectively configured to selectively form the electron beam so that the beam provides an electron distribution within the focal point which maintains each point within the focal spot at substantially the same temperature.
In a preferred embodiment, the filament has an associated axis and the focal spot has length and width dimensions. The length dimension is measured between two focal spot end points along a direction which is parallel to the axis, and the width dimension is measured along a direction which is orthogonal to the filament axis and the length direction. The filament and cathode cup are respectively configured to form the beam so that the beam defines a focal spot having width dimensions at its end points which are substantially less than the focal spot width at a location midway between the two end points. Preferably also, the target track comprises tungsten, the anode comprises a rotatable disk formed of a refractory metal, and a potential difference on the order of 100 kilovolts is maintained between the cathode and the anode to produce X-rays.
In a useful embodiment, the cathode cup is provided with a planar surface having a channel formed therein, and the filament comprises a helical filament disposed for insertion into the channel, the helical filament having a central portion and opposing end portions. The filament is selectively curved, so that its end portions are recessed deeper into the channel than the central portion thereof, relative to the planar surface of the cup.
In another useful embodiment, the filament comprises a linear helical filament having a central portion and opposing end portions. The cathode cup is provided with a selectively curved surface having a channel formed therein, the filament being inserted into the channel so that the opposing end portions of the filament are recessed further into the channel than the central portion thereof, as a result of the curvature of the cup.